Leanring Kotlin Coroutines

Suspending functions, like withContext, can only be called from other suspending functions or inside a coroutine builder.

对于Kotlin的Coroutines，不应该参考维基百科的协程定义。

Kotlin’s coroutines are a language-level feature designed for structured concurrency, while operating system coroutines are a low-level mechanism for cooperative multitasking provided by the operating system or user-level libraries.